ventillation is even more important then the amount of light you have. if you just recycle air in your grow room, you will use up all the CO2 which is a large contributer to bud mass. This is why people who use additional CO2 can get 40% increases in mass. But most importantly, you need fresh air thats constantly cycling in and moving around the grow room (specifically around the leaves and branches) or it needs to be supplemented with CO2, A/C, etc.

having your lights to far from the plant will also cause whispy and light buds, but nutrients, stress, and specific strain can all cause an effect on the density. the #1 rule that every very experienced grower has taught me is ventillation ventillation ventillation.